Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
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HOMESERVICE MANUALSGMC1999FORWARD CONTROL 5.7 R, AUTOMATICREPAIR AND DIAGNOSISEXTERNAL PAGESDIFFERENT CARSECTION 24 (BODY CONTROL MODULES)SELF-DIAGNOSTIC SYSTEMENGINE ELECTRICAL SYSTEM DIAGNOSTIC SYSTEM CHECK
1999 GMC Forward Control 5.7 R, Automatic
Engine Electrical System Diagnostic System Check
1999 GMC Forward Control 5.7 R, AutomaticSECTION Engine Electrical System Diagnostic System Check
WARNING: This page does not describe the selected car, but rather 8 other vehicles, including the 2002 GMC Savana, 2002 Chevrolet Chevy Express, 2001 GMC Savana Special, 2001 GMC Savana Camper Special, and 2001 GMC Savana. However, it is still accessible from the selected car via links, so may be relevant.
- Ensure battery is okay and fully charged. If battery is okay, go to next step. If battery is not okay, repair or replace as necessary.
- Turn ignition switch to OFF position. Connect scan tool to Data Link Connector (DLC), located below steering column. Turn ignition switch to RUN position. If scan tool powers up, go to next step. If scan tool does not power up, go to TEST A: SCAN TOOL DOES NOT POWER UP under SYSTEM TESTS.
- Using scan tool, attempt to communicate with all modules on class 2 serial communications network. If scan tool communicates with all modules, go to next step. If scan tool does not communicate with all modules, go to TEST B: SCAN TOOL DOES NOT COMMUNICATE WITH CLASS 2 DEVICE under SYSTEM TESTS.
- Select DISPLAY DTCs function on scan tool for each module. If scan tool displays any DTCs go to next step. If can tool does not display any DTCs, repair by symptom. See TROUBLESHOOTING - NO CODES - TRUCKS article in ENGINE PERFORMANCE.
- If scan tool displays DTCs which begin with "U", go to TEST B: SCAN TOOL DOES NOT COMMUNICATE WITH CLASS 2 DEVICE under SYSTEM TESTS. If other DTCs are displayed, perform appropriate test. See DIAGNOSTIC TROUBLE CODE DEFINITIONS .
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.